This invention relates generally to superconducting magnets and more particularly to high field magnets for use, e.g., in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) devices.
Technology in high field NMR and MRI magnets has been perfected in stages over 20 years. Starting with non-homogeneous common non-persistent magnets, the technology of NMR magnet design and construction, particularly uniformity correction, is now well understood. It is well known in NMR magnet construction that introduction of Nb.sub.3 Sn material led to significant uniformity degradation because of the softness of a glass cloth insulation which was used in such magnet construction. This softness resulted in a lack of precision in placing magnet turns and a problem that necessitated a better understanding of correction of high order inhomogeneities. Having gained this knowledge, new HTS materials, with much higher critical fields, suggest avenues for their future applications in high field magnets. HTS materials offer unique high field capability at a cryogenic temperature of 4.2 K, the boiling point of liquid helium.
In U.S. Pat. No. 5,310,705 to Mitlitsky and Hoard, a high field magnet generating a field greater than 4 Tesla is fabricated from high critical temperature superconducting ceramic thin films. The high field magnet is made of stackable disk shaped substrates, which are coated with high critical temperature superconducting ceramic thin films. The films are electrically interconnected in series and are deposited on one or both sides of the disk substrates in a spiral configuration with variable line widths to increase the field.
However, 17.6 Tesla, not 4.0 Tesla, is state-of-the-art for NMR devices. These strong fields are achieved using low temperature superconducting materials such as the niobium tin composition, mentioned above, in the form of solenoids that have good stability and uniformity when operating in a persistent mode. Operation is at 4.2 K, which is achieved by boil off of liquid helium in many applications.
While a magnet for an NMR device operating at 17.6 Tesla and 4.2 K is a remarkable achievement in the art, there is the desire and/or need to produce higher field strengths, especially with magnets of the same or reduced size. It is understood that for a particular superconducting magnet, the current, and field strength are directly related. However, it is necessary in order to achieve stability in a persistent mode that the operating current in the magnet coils be substantially less than the critical current of the superconducting material. In the art, it is stated that the superconductors operate at a percentage of their "short sample" performance. Typical state-of-the-art high field NMR magnets operate at between 60%-70% of the critical current, i.e., the ratio of operating current i to critical current Ic is between 0.6 and 0.7. To raise the operating current closer to the critical current substantially increases the risk of undesirable quenching of the superconducting magnet and the risk of other instabilities in the magnetic field, as are known to those skilled in the art.
Field strength can be increased with stable performance by reducing the operating temperature of the magnet, which lower temperature permits higher values of operating current at the same time maintaining a substantial safety margin relative to the critical current. For example, the field strength of a Nb.sub.3 Sn 17.6 Tesla magnet at 4 K can be increased by operating, e.g., at 2.2 K, and this lower temperature has been used in superconducting magnet systems known in the art.
However, when operating at 2.2 K, the relative simplicity of using a liquid helium boil-off refrigerating system at 4.2 K must give way to a vapor reduction system using a motorized vacuum pump, heat exchangers, throttle devices, etc. as are known in the art. On the other hand, a Nb.sub.3 Sn magnet cannot maintain the increased field at 4.2 K and provide stability and assured persistence.
Superconducting magnets are needed that for a given operating temperature provide higher magnetic fields in the persistent mode than presently known magnets operating at the same temperatures. Also needed are superconducting magnets that provide the same or greater magnetic fields as presently known magnets, however, operating at a higher superconducting temperature.